1. Field of the Invention
This technology relates generally to calibrating plenoptic imaging systems.
2. Description of the Related Art
The plenoptic imaging system has recently received increased attention. It can be used to recalculate a different focus point or point of view of an object, based on digital processing of the captured plenoptic image. The plenoptic imaging system also finds application in multi-modal imaging, using a multi-modal filter array in the pupil plane of the primary imaging module. Each filter is imaged at the sensor, effectively producing a multiplexed image of the object for each imaging modality of the filter array. Other applications for plenoptic imaging systems include varying depth of field imaging and high dynamic range imaging.
However, the architecture of a plenoptic imaging system is different from that of a conventional imaging system, and therefore requires different calibration and processing procedures. Several challenges are found in the processing of plenoptic images. First, the alignment of the microlens array is never perfect and the effect of rotation of the microlens array is quite observable. This rotation introduces a large amount of difficulties for image reconstruction because the data points do not fall onto a regular sampling grid. Second, for a modular system architecture, different detector arrays and microlens arrays could be used based on different applications. Manual determination of parameters, such as center of lenslet, pixels under each lenslet, spacing of lenslets, etc., is difficult and time consuming. Third, for different applications, different objective lenses could be used. The parameters necessary for image reconstruction are different when the plenoptic image data are taken with different objective lenses or at different focal settings.
Current techniques for calibrating plenoptic imaging systems involve imaging simple camera targets. These targets can be uniform white targets, grids, checkerboards, etc. The targets are typically illuminated with spatially incoherent light. For the case of the uniform target, light scattered from the target uniformly illuminates the aperture of the plenoptic camera's primary lens. The exit pupil from the primary lens is then imaged onto the sensor array by each microlens. Each “superpixel” in the sensor array is an image of the primary lens aperture (the exit pupil). Current image processing techniques analyze the size and position of these superpixels to determine a calibration for image reconstruction.
A drawback of current techniques is that they neglect possible aberrations in the exit pupil images collected by the sensor. The superpixels can be distorted, shifted, vignetted, or cropped. The aberrations can be field dependent. These aberrations can cause an incorrect calibration to be produced. Examples of systems where the exit pupil image changes with field position include systems that are non-telecentric in image space (the chief rays are not perpendicular to each microlens, and not parallel to the optical axis), systems with distortion (change in magnification of the exit pupil with field position), systems with field curvature, systems with vignetting surfaces, etc.
Thus, there is a need for better calibration techniques for plenoptic imaging systems.